This invention relates to a method of building low cost tooling and to a method of forming low cost precision plastic panels therein having excellent reproducibility and surface finish. The mold-building technique provides accurate molds which can be made at low cost, which provide means for placement of load-spreading metal attachment inserts, and which provide dimensional precision using low capacity clamping means when used with the subject part forming method. The part-forming technique provides large thinwall panels having excellent dimensional accuracy, excellent surface finish, and no warpage. Because of the technique of forming the panels by foamingin-place of a reactive polyurethane-chopped glass fiber mixture; this process permits design of low strength precision molds and, consequently, use of low clamping force presses for molding. Since they are subject to low clamping and separating forces, the molds may be manufactured allowing for accurate reproduction of surface details without concern for high strength. This provides the advantage of the present part forming/mold making system.
Manufacture of large thin wall glass fiber reinforced plastic panels generally requires high strength precision molds situated in high capacity presses to resist the enormous separating forces experienced by the mold halves during injection or resin transfer molding. This is necessitated by the relatively high viscosity at which transfer molding and injection molding are accomplished and by the high pressure required to force the resin through the narrow passages of the mold.
The present invention requires only accurate mold surfaces and sufficient, low, clamping forces to resist separating forces attributable to the foaming action of the pre-metered polyurethane foam-chopped glass fiber mixture as it expands to fill the mold. Because of this "self-expanding" mold filling feature; the reactive resin/glass fiber mixture is only "guided" by the mold to achieve the accuracy and mold-replication required by the product and, hence, does not demand the high strength and gating and riser provisions required by molds used in transfer and injection molding.
Automobile roof and door restyling panels are commonly formed from glass fiber reinforced polyester resins by spraying into an open mold. The quality of the panel produced thereby depends on the skill of the operator and may thus vary to an unacceptable degree. One advantage of the open mold spray process is that the mold need not be expensive nor possess high strength. Parts produced, however, often require excessive rework and refinishing. Resin transfer molding, cold press molding, and injection molding require high strength precision-matched mold sets but are capable of excellent reproducibility in part quality. Mold costs are high, mold construction lead times are long and the processes also require high capacity precision presses. These factors mitigate against use of the latter three processes except for very high production quantities. Since automobile restyling trends are subject to changes in tastes of consumers, it is advantageous for small manufacturers to be able to respond rapidly to such changes.
The present invention permits such rapid response at a small fraction of the cost of other processes.